Interest has developed in recent years in the protection of currency and other value documents from counterfeiting by using security devices that include interference filters. The color variations available from an interference filter cannot be duplicated by a copying machine. Interference filters are known from Tripp, U.S. Pat. No. 2,590,906. Typically, an interference filter includes a reflective stack having a reflective metal film on a smooth substrate, a transparent dielectric spacer layer atop the reflective metal layer and a partially reflective metal layer atop the spacer layer. In some cases a transparent protective coating can be applied over the reflective stack. This coating does not form part of the interference filter. The interference filter can also include an optical element, e.g., a lens or series of lenses.
When an incident light beam encounters the partially reflective layer one fraction of the light is reflected and another fraction passes through the partially reflective layer into the dielectric layer. The transmitted fraction is reflected by the reflective layer and retransmitted through the dielectric layer. A fraction of the reflected light passes through the partially reflective layer and may constructively or destructively interfere with the incoming light.
The optical thickness of the transparent dielectric spacer layer is often a small multiple of a quarter wavelength of light for constructive interference. Thus, when light is reflected from the interference filter, light of appropriate wavelength has the reflected and transmitted fractions in phase for constructive interference. Light of other wavelengths may have at least partial destructive interference. Thus, when a reflective interference filter is observed in white light, its reflection has a characteristic color.
The color reflected from the filter depends on the effective optical thickness of the interference filter. When the filter is observed with light at normal incidence, a certain color, e.g., blue, is seen. When the angle of incidence and reflection from the interference filter is more acute, the effective optical thickness is shorter. Thus, when the interference filter is observed at an angle nearer grazing incidence, a shorter wavelength color, for example purple, is observed. The characteristic change of color is thus dependent on the viewing angle.
In some instances the security device can include an image, an optical element, or both. The device can be designed so that the image will only be visible at predetermined observation angles. The resulting effects can provide useful features including evidence of tampering, security authentication or positional information. For example, the visibility and conspicuity of an object can be enhanced by making the color of the light dependent on information about the object, such as its orientation to the light source and the object's color shifting properties.
Dielectric spacer layers used in interference filters have been prepared from acrylate monomers. The spacer layer may require a pretreatment or a primer between the spacer layer and the reflective metal layers to obtain sufficient bonding strength between the layers. There is a need for dielectric layers that have improved bond strength with the metal layer(s) and which require no pretreatment or a minimum number of pretreatment steps.